Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semi-conductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as P-type and N-type according to whether they comprise semi-conductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semi-conductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semi-conductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and injecting holes or electrodes. For example, a P-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO level of the semi-conductive material can enhance hole injection and acceptance. In contrast, an N-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO level of the semi-conductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form thin film transistors. When an organic material is used as the semi-conductive material in such a device, it is known as an organic thin film transistor.
Various arrangements for organic thin film transistors are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semi-conductive material disposed therebetween in a channel region, a gate electrode disposed adjacent the semi-conductive material and a layer of insulting material disposed between the gate electrode and the semi-conductive material in the channel region.
An example of such an organic thin film transistor is shown in FIG. 1. The illustrated structure may be deposited on a substrate 1 and comprises source and drain electrodes 2, 4 which are spaced apart with a channel region 6 located therebetween. An organic semiconductor (OSC) 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4. An insulating layer 10 of dielectric material is deposited over the organic semi-conductor 8 and may extend over at least a portion of the source and drain electrodes 2, 4. Finally, a gate electrode 12 is deposited over the insulating layer 10. The gate electrode 12 is located over the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
The structure described above is known as a top-gate organic thin film transistor as the gate is located on a top side of the device. Alternatively, it is also known to provide the gate on a bottom side of the device to form a so-called bottom-gate organic thin film transistor.
An example of such a bottom-gate organic thin film transistor is shown in FIG. 2. In order to more clearly show the relationship between the structures illustrated in FIGS. 1 and 2, like reference numerals have been used for corresponding parts. The bottom-gate structure illustrated in FIG. 2 comprises a gate electrode 12 deposited on a substrate 1 with an insulating layer 10 of dielectric material deposited thereover. Source and drain electrodes 2, 4 are deposited over the insulating layer 10 of dielectric material. The source and drain electrodes 2, 4 are spaced apart with a channel region 6 located therebetween over the gate electrode. An organic semiconductor (OSC) 8 is deposited in the channel region 6 and may extend over at least a portion of the source and drain electrodes 2, 4.
One of the challenges with all organic thin film transistors is to ensure a good ohmic contact between the source and drain electrodes and the organic semiconductor (OSC). This is required to minimize contact resistance when the thin film transistor is switched on. A typical approach to minimize extraction and injection barriers, for a p-channel device, is to choose a material for the source and drain electrodes that has a work function that is well matched to the HOMO level of the OSC. For example, many common OSC materials have a good HOMO level matching with the work function of gold, making gold a relatively good material for use as the source and drain electrode material. Similarly, for an n-channel device, a typical approach to minimize extraction and injection barriers is to choose a material for the source and drain electrodes that has a work function that is well matched to the LUMO level of the OSC.
One problem with the aforementioned arrangement is that a relatively small number of materials will have a work function which has a good match energy level match with the HOMO/LUMO of the OSC. Many of these materials may be expensive, such as gold, and/or may be difficult to deposit to form the source and drain electrodes. Furthermore, even if a suitable material is available, it may not be perfectly matched for a desired OSC, and a change in the OSC may require a change in the material used for the source and drain electrodes.
One known solution is to provide a thin self-assembled dipole layer on the source and drain electrodes to improve the energy level matching. While not being bound by theory, a thin self-assembled dipole layer may provide a field which shifts the energy levels of the material of the source/drain electrodes and/or the energy levels of the OSC near the source/drain electrodes to improve energy level matching between the OSC and the material of the source/drain.
Although the use of a self-assembled dipole layer can improve matching between the energy levels of the source/drain material and the OSC, the energy levels can only be shifted by a few tenths of an electron volt using this technique. As such, the type of material used for the source and drain electrodes is still relatively restricted. It would be advantageous to be able to use a wide range of materials for the source and drain so that materials can be chosen for their process compatibility. Another problem is that if the thin self-assembled dipole layer is disposed not only on the source/drain electrodes, but also in the channel region, then the performance characteristics of the OSC in the channel region can be adversely affected.
Several other approaches have been used in the prior art in order to improve organic thin film transistor performance.
US 2005/133782 discloses doping of source/drain palladium metal by using of benzo-nitrile or substituted benzo nitriles such as Tetracyanoquinodimethane (TCNQ) in order to facilitate the transfer of charge between the organic semiconductor and the source/drain electrode surface. In contrast to the dipole layers discussed above which merely alter the energy levels of the OSC and/or source and drain using a field effect, the benzo-nitriles chemically dope the OSC by accepting electrons (p-doping). As such, the conductivity of the OSC near the electrodes is increased and charge transfer is facilitated to a much larger extent than utilizing the aforementioned dipole layers.
J. Am. Chem. Soc., 2006, 128, 16418-16419 also discloses use of TCNQ on either Ag or Cu contacts to locally dope a pentacene OSC giving good transistor properties.
The nitriles are used directly in the aforementioned prior art without being functionalized with groups specially designed for attachment to the source/drain metal. It is described that the dopant nitrile groups can themselves bond to source/drain palladium metal and unbonded dopant can be removed by washing to leave the dopant nitrile groups attached to the source/drain but not in the channel.
The present applicant has found that it is advantageous to improve the binding of dopant moieties such as TCNQ to the source/drain electrodes by providing an attachment moiety bonded to the dopant moiety. This is described in the applicant's earlier application GB-A-0712269.0. GB-A-0712269.0 also disclosed that fluorinated derivatives of TCNQ such as tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) could be used as these dopant moieties were found by the applicant to be particularly good at accepting electrons from the OSC due to its very deep LUMO.
It is an aim of certain embodiments of the present invention to provide an improved organic thin film transistor and an improved method of treating source/drain electrodes in order to provide a good ohmic contact between the source/drain electrodes and the organic semiconductor material in an organic thin film transistor.
It is a further aim of the present invention to provide a method of forming an organic thin film transistor with good ohmic contact by a solution processing method.